The present invention relates to submerged filters to be filled with during a normal filtering operation granular material in which a liquid to be treated and flowing from bottom to top is brought into contact with a gas likewise flowing from bottom to top.
These filters are used in particular for the biological purification of sewage by "fixed cultures" and, more generally, any time when a gas and a liquid flow in an ascending co-current manner through a completely immersed granular material.
The conventional process for the biological purification of sewage by fixed cultures consists in causing this sewage to seep through a granular mass colonized by purifying micro-organisms capable of decomposing by microbial action the organic matter the sewage carries. This biodegradation can only be accomplished if these microorganisms receive a supply of oxygen that is sufficient for their energy needs, their reproduction and their endogenous respiration. The oxygen source usually consists of atmospheric air, but pure oxygen or air enriched with oxygen can likewise be employed.
Because of the supply by the sewage to be treated of materials in suspension, and in particular because of the generation of bacterial mass inherent to the process, there occurs over time a filling of the intergranular space and thereby a clogging of the filtering environment which then will have to be cleaned. Hence, the pieces of apparatus for carrying out the purification process must include an underdrainage system or supporting floor for the granular material and which is equipped with nozzles of a type known from the prior art to ensure an efficient washing by water and air of this granular material.
The main difficulty that arises when carrying out the process in an apparatus operating with an ascending current lies in the necessity of distributing homogeneously throughout all of the granular contact mass the water within which there is generated, under aerobic conditions, a considerable amount of bacteria and the air necessary for carrying out the process, whereby the water and air flow rates are likely to fluctuate over time. This distribution of the two fluids is indispensable for the proper implementation of the process, since an inadequate distribution of the water to be treated will result in an improper utilization of all of the space of the apparatus and, consequently, in a degradation of the purification results, and an inadequate distribution of the air will lead to the creation of anaerobic zones to the point where the efficiency of the apparatus is lowered.
To ensure this distribution, at least two approaches have been proposed in the past.
A first approach consists in disposing above the underdrainage system or floor of the granular environment a branched, perforated header for distributing the treating air. This approach has at least two disadvantages:
when the flow of treating water is low or when the plant is at a standstill, this header is partially or totally filled with raw material, which leads to a relatively long duration clogging of the distribution openings owing to the bacterial generation therein. PA1 the presence of a network of pipes above floor or underdrainage system for supporting the granular material interferes with the flow of the fluids during washing, thereby lowering the efficiency of the apparatus. PA1 the construction of an apparatus thus designed is complicated and entails great costs.
Theoretically, a second approach enables one to make up for the first drawback of the first approach. It consists in distributing the water to be treated above the floor through a branched header and to introduce the treating air below the floor by means of nozzles. Since the flow rate of the treating air can be ten times lower than that of the wash air, its distribution can only be accomplished by a fraction of the total number of nozzles each of which includes, in the upper part of its tube that emerges some fifteen centimeters below the floor, a sized hole that creates, during the passage of air, a loss of charge such as to form below the floor an air cushion with a uniform thickness and which feeds the nozzles for distributing the treating air. Since the water that fills the space defined between the floor and the bottom of the apparatus is clear water, there is, in theory, below the floor between the raw water and the air, no contact that is likely to lead to bacterial generation and that can lead to the clogging of the air distribution holes.
In essence, it has been found that, on account of the emergence of an "air lift" phenomenon, there arose, concurrently with a lifting of clear water through nozzles for distributing treating air, a lowering in said space of an equal flow rate of raw water distributed above the floor by the branched header, so that the clear water was quickly replaced by raw water, with the result that a biomass was generated in the tubes of the nozzles for the distribution of treating air. This leads to the gradual clogging and plugging of the openings for passage of the air.